The invention relates to the use of an apparatus with at least one shearing module for essentially salt-free coagulation of plastics dispersions, and also to the process carried out with this apparatus.
Many polymers are prepared by homo- or copolymerization of suitable monomers in a liquid medium, e.g. by emulsion, miniemulsion or microsuspension poly-merization. Here, the polymer precipitates in the form of a usually aqueous dispersion of solid, from which the polymer has to be separated out, unless the dispersion is to be used as such.
The polymers are usually separated out from the dispersion by coagulation. There is a wide variety of different known methods for this. For example, dispersions can be coagulated by adding strong electrolytes. This is mostly done using salts which contain polyvalent cations, such as Ca2+, Mg2+ or Al3+. A disadvantage of this method is that relatively large amounts of precipitating agents remain in the product and impair important product properties. Downstream washing of the precipitated polymer with large amounts of water is therefore necessary, and this causes problems in terms of costs and the environment. Another disadvantage of precipitation with electrolytes is that the precipitated product is frequently produced as a clump which comprises unprecipitated material or excess precipitating agent, or as very finely divided material difficult to separate out by sedimentation or filtration.
It has also become known that polymer dispersions can be coagulated by subjecting them to high shear forces. Here, the respective polymer dispersion is subjected to high shear forces until the polymer particles agglomerate. If the solids content of the polymer is above 20%, the polymer coagulated in this way can become pasty to crumbly.
DE-A-196 54 169 discloses a process for coagulating graft-rubber dispersions, where coagulation is brought about using shear-precipitation in a stator-rotor arrangement. Both the stator and the rotor, which rotates within the stator, have slots through which the dispersion is passed radially from the inside to the outside as a result of the rotation of the rotor. The shear to which the dispersion is subjected here is strong enough for it to coagulate.
DE-A-29 17 321 discloses a process for separating out, from an aqueous emulsion, polymers which have a softening range above 100xc2x0 C., where the aqueous emulsion is coagulated in an extruder by shearing and/or heating to temperatures above the softening range of the polymer, and the coagulated material is then melted and discharged hot from the extruder, under pressure. The water is then separated out in a subsequent step. The process is very energy-intensive and requires a counter-rotating non-intermeshing twin-screw extruder for the precipitation. In addition, ammonium acetate is used as auxiliary to accelerate the coagulation, and this is undesirable for environmental reasons.
U.S. Pat. No. 3,821,348 describes a process in which acrylonitrile-copolymer dispersions or acrylonitrile-graft-polymer dispersions with a high acrylonitrile content and a very low content of elastomeric butadiene-acrylonitrile rubber are coagulated to give a paste, using a Waring mixer as the shearing apparatus, and then extruded through a fine die to give thin lengths and passed into hot water. The product is then washed, dried and finally shaped into lengths in a compression molder at 150xc2x0 C.
It is an object of the present invention, in the light of this prior art, to provide an apparatus and a process for coagulating plastics dispersions or rubber dispersions, with which cost-effective coagulation of dispersions of this type becomes possible without adding chemical coagulants.
We have found that this object is achieved by using an apparatus with at least one shearing module which has a stator and a rotor arranged within the stator, where the surfaces facing toward one another in the stator and in the rotor are in each case smooth, or at least the rotor exhibits a structure formed on its surface and facing from this in the direction of the stator, and between the stator and the rotor there is a gap of predetermined gap width.
For the purposes of the present invention, xe2x80x9cgapxe2x80x9d is a very general and inclusive term for any desired space between rotor and stator. The predetermined gap width may therefore also include the flight depth, defined as (outer diameter of a screw minus the diameter of the screw root)/2.
This apparatus has proven very reliable in the essentially salt-free coagulation of plastics dispersions or rubber dispersions. It is fundamentally very simple in construction, and no susceptibility to clogging has been found. If desired, additional conveying modules may be used to convey the dispersion to be coagulated to the apparatus and away from the apparatus after coagulation has taken place. However, the apparatus may also be freely operated without conveying modules of this type. In particular, there is no requirement to use, for example, pressure vessels or pumps to ensure the presence of a certain pressure in advance in order to supply the apparatus with the dispersion to be coagulated.
For the purposes of the present invention, plastics dispersions are dispersions in which the homo- and/or copolymers have a glass transition temperature above 0xc2x0 C., whereas the glass transition temperatures for rubber dispersions are below 0xc2x0 C.
The predetermined gap width may be constant, but may also in each case vary within each of the one or more shearing modules. The diameter of the rotor here may decrease or increase in the direction of conveying. This decrease or increase in the diameter in the direction of conveying may occur more than once.
It has proven advantageous for the diameter of the rotor to diminish in the direction of conveying, or for the predetermined gap width to decrease in the direction of conveying.
The rotor may have a toothed-wheel structure, the rows of teeth in which have a circular arrangement radially around the rotor. If desired, the stator may have one or more approximately complementary rows of teeth. In this arrangement the coagulation mechanism is different from that with smooth surfaces of the stator and rotor. Whereas in that case coagulation takes place as a result of exposure to a continuous shear field, the use of a stator-rotor combination whose rotor has a surface structure, or of a stator-rotor combination with complementary toothed wheel or, respectively, rows of teeth gives a constantly repeating shear stress. The dispersion experiences a reduction in pressure once one of the rotor teeth has passed by the stator, only to be subjected again to strong shear at the next tooth which follows. This arrangement gives very intensive shear action. Depending on the requirements relating to the dispersion to be coagulated, a selection may therefore advantageously be made between a smooth stator-rotor system, i.e. a stator-rotor system with a smooth surface, and one in which at least the rotor surface has a toothed-wheel structure.
The rows of teeth on the stator and on the rotor may be approximately rectangular. They may also have an approximately star-shaped arrangement on the rotor. A helical arrangement of teeth is also possible, but for this there can be no complementary shaping of the stator.
Upstream and/or downstream of the shearing module of the apparatus used according to the invention, there may be a conveying screw with one or more flights, preferably arranged on the same shaft as the shearing module. The feeding and transport of the dispersion to be coagulated in the apparatus, and also the to discharge of the coagulated dispersion, can be made to occur of their own accord if a conveying screw is used.
The gap width may vary within a relatively wide range, depending on the dispersion to be coagulated and the product quality desired. Gap widths of from about 0.05 to 20 mm give good results, and even if the gap width is in the lower region no susceptibility to clogging of the apparatus is found. Typical gap widths which may be mentioned for a stator-rotor arrangement with a structured surface are from 0.05 to 20 mm, while for a stator-rotor combination with a smooth surface they are within the range from about 0.3 to 10 mm.
In a preferred use, the shearing module is a screw module, the screw of which forms the rotor. Particular preference is given here to a screw in which the diameter of the screw root increase in the direction of conveying. This of necessity results in a decrease in the predetermined gap width, i.e. in the flight depth in this case, where the rotor is a screw. Such a screw module is named a screw having an increasing root.
The shearing module in the form of a screw module simplifies the construction of the apparatus used according to the invention, since the screw can serve simultaneously as conveying screw and as shearing module. It has been found that this arrangement can also considerably reduce the drive power used to transport the plastics dispersion or rubber dispersion to be coagulated, where appropriate in the partially coagulated state, through the shearing apparatus.
The invention also provides a process for essentially salt-free coagulation of plastics dispersions or rubber dispersions using the apparatus described in greater detail above. In this process, the dispersion is passed through the gap between stator and rotor and is precipitated and subjected to a predetermined shear rate and shear deformation by rotation of the rotor.
This type of shear precipitation can be carried out without the addition of strong electrolytes, as coagulants, to be dispensed with. The process can also be carried out continuously.
If the shearing gap is smooth, the decisive parameters for the quality of the precipitation are the shear rate and, respectively, the shear deformation.
In a preferred embodiment, the shear rate is from about 100 to 100,000 sxe2x88x921 and the shear deformation is from about 1 to 100,000.
The rotor may rotate at a rotation rate of from about 50 to 10,000 rpm, preferably from about 200 to 8000 rpm. For a stator-rotor combination whose surface has a toothed-wheel structure, rotation rates of up to 8000 rpm have also proven successful.
The novel process may be used, for example, for coagulating plastics dispersions and preferably rubber dispersions, composed, for example, of:
from 60 to 100 parts by weight, based on the total weight of the finished dispersion, of at least one monomer (main monomer) capable of being incorporated by polymerization,
from 0 to 35 parts by weight, preferably from 0 to 20 parts by weight, of at least one functional monomer (comonomer), and
from 0 to 5 parts by weight, preferably from 0 to 3 parts by weight, of an xcex1,xcex2-unsaturated mono- or dicarboxylic acid.
The main monomer has preferably been selected from the group consisting of:
esters preferably made from xcex1,xcex2-monoethylenically unsaturated mono- or dicarboxylic acids having from 3 to 6 carbon atoms, for example acrylic acid, methacrylic acid, maleic acid, fumaric acid or itaconic acid, and from in general C1-C12 alkanols, preferably C1-C8 alkanols and in particular C1-C4 alkanols.
Particular esters of this type are methyl, ethyl, n-butyl, isobutyl, tert-butyl and 2-ethylhexyl acrylates and the corresponding methacrylates;
vinylaromatic compounds, such as styrene, xcex1-methylstyrene, xcex1-chloro-styrene and vinyltoluenes;
vinyl esters of C1-C18 mono- or dicarboxylic acids, for example vinyl acetate, vinyl propionate, vinyl n-butyrate, vinyl laurate and vinyl stearate;
butadiene.
Particularly preferred main monomers are methyl methacrylate, methyl acrylate, n-butyl methacrylate, tert-butyl methacrylate, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, styrene and vinyl acetate.
Particularly suitable monomers are:
linear 1-olefins, branched-chain 1-olefins and cyclic olefins, e.g. ethene, propene, butene, isobutene, pentene, cyclopentene, hexene, cyclohexene, octene, 2,4,4-trimethyl-1-pentene, if desired mixed with 2,4,4-trimethyl-2-pentene, C8-C10 olefins, 1-dodecene, C12-C14 olefins, octadecene, 1-eicosene (C20), C20-C24 olefins; oligoolefins prepared with metallocene catalysis and having a terminal double bond, e.g. oligopropene, oligohexene and oligooctadecene; polyolefins prepared by cationic polymerization with a high proportion of a-olefin, for example polyiso-butene. However, it is preferable for no ethene and no linear 1-olefin to be incorporated into the polymer.
Acrylonitrile, methacrylonitrile
Vinyl and allyl alkyl ethers having from 1 to 40 carbon atoms in the alkyl radical, where the alkyl radical may also have other substituents, such as hydroxyl, amino or dialkylamino, or they may have one or more alkoxylate groups, for example methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, isobutyl vinyl ether, 2-ethylhexyl vinyl ether, vinyl cyclohexyl ether, vinyl 4-hydroxybutyl ether, decyl vinyl ether, dodecyl vinyl ether, octadecyl vinyl ether, 2-(diethylamino)ethyl vinyl ether, 2-(di-n-butyl-amino)ethyl vinyl ether, methyldiglycol vinyl ether, and also the corresponding allyl ethers, and mixtures of these.
Acrylamides and alkyl-substituted acrylamides, e.g. acrylamide, methyl-acrylamide, N-tert-butylacrylamide, N-methyl(meth)acrylamide.
Sulfo-containing monomers, e.g. allylsulfonic acid, methallylsulfonic acid, styrenesulfonate, vinylsulfonic acid, allyloxybenzenesulfonic acid, 2-acryl-amido-2-methylpropanesulfonic acid, and the appropriate alkali metal salts or ammonium salts of these, and mixtures of these, and also sulfopropyl acrylate, sulfopropyl methacrylate.
C1-C4-Hydroxyalkyl esters of C3-C6 mono- or dicarboxylic acids (see above), in particular of acrylic acid, methacrylic acid or maleic acid, or derivatives of these alkoxylated with from 2 to 50 mol of ethylene oxide, propylene oxide, butylene oxide, or mixtures of these, or esters, with the acids mentioned, of C1-C18 alcohols alkoxylated with from 2 to 50 mol of ethylene oxide, propylene oxide, butylene oxide or mixtures of these, for example hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, 1,4-butanediol monoacrylate, ethyldiglycol acrylate, methylpolyglycol acrylate (11 EO), (meth)acrylates of C13/C15 oxoalcohols reacted with 3, 5, 7, 10 or 30 mol of ethylene oxide, or mixtures of these.
Vinylphosphonic acid, dimethyl vinylphosphonate and other phosphorus-containing monomers.
Alkylaminoalkyl (meth)acrylates, alkylaminoalkyl(meth)acrylamides or quaternization products of these, for example 2-(N,N-dimethylamino)ethyl (meth)acrylate, 3-(N,N-dimethylamino)propyl (meth)acrylate, 2-(N,N,N-trimethylammonium)ethyl (meth)acrylate chloride, 2-dimethyl-aminoethyl(meth)acrylamide, 3-dimethylaminopropyl(meth)acrylamide, 3-trimethylammoniumpropyl(meth)acrylamide chloride.
Allyl esters of C1-C30 monocarboxylic acids.
N-Vinyl compounds, such as N-vinylformamide, N-vinyl-N-methylformamide, N-vinylpyrrolidone, N-vinylimidazole, 1-vinyl-2-methylimidazole, 1-vinyl-2-methylimidazoline, N-vinylcaprolactam, vinylcarbazole, 2-vinylpyridine, 4-vinylpyridine.
Diallyldimethylammonium chloride, vinylidene chloride, vinyl chloride, acrolein, methacrolein.
Monomers containing 1,3-diketo groups, for example acetoacetoxyethyl (meth)acrylate and diacetoneacrylamide, monomers containing urea groups, for example ureidoethyl (meth)acrylate, acrylamidoglycolic acid, methyl methacrylamidoglycolate.
Monomers containing silyl groups, for example trimethoxysilylpropyl methacrylate.
Monomers containing glycidyl groups, for example glycidyl methacrylate.
Dispersions suitable for the novel coagulation process, besides normal emulsions, are in particular graft-rubber dispersions which have been prepared in aqueous emulsion at least in the final stage of the graft polymerization, by grafting of the elastomers with the monomers for the graft shell.
For the purposes of the present invention, graft rubbers are in particular those graft polymers in which monomers forming hard thermoplastics, for example in particular styrene, acrylonitrile and/or methyl methacrylate, are grafted as a graft shell onto particle cores made from soft rubber. This is done by polymerizing or copolymerizing the monomers for the graft shell in the presence of the rubber particles. Suitable soft rubbers are elastomeric polymers and/or copolymers with glass transition temperatures below xe2x88x9210xc2x0 C., preferably below xe2x88x9230xc2x0 C. Particularly suitable polymers are elastomeric 1,3-diene homo- and copolymers, such as homo- and copolymers of butadiene, isoprene or chloroprene, preferably butadiene rubber, and also elastomeric acrylate homo- and/or copolymers with the low glass transition temperatures mentioned. Preferred polymers for the graft rubbers coagulated according to the invention are elastomeric acrylate polymers and 1,3-diene homo- and copolymers, for example homo- and copolymers of C4-C8-alkyl acrylates, in particular of n-butyl acrylate and/or 2-ethylhexyl acrylate. Examples of preferred comonomers for the alkyl acrylates are crosslinking monomers having at least two nonconjugated Cxe2x95x90C double bonds, for example diallyl maleate, diallyl phthalate, diacrylates and dimethacrylates of diols, such as 1,4-butanediol or 1,6-hexanediol, etc., and also allyl methacrylate and dihydrodicyclopentadienyl acrylate, used in particular in amounts of from 0.5 to 10% by weight of the total amount of monomers in the elastomer preparation, and also polar monomers, such as acrylic acid, methacrylic acid, maleic anhydride, acrylamide, methacrylamide, N-methylolacrylamide and -methacrylamide, and alkyl ethers of these. The proportion of the elastomers in the graft rubber is generally from 30 to 85% by weight. The novel process may be used without difficulty to coagulate graft rubbers whose elastomer proportion is more than 30% by weight, based on the total solids content.
Suitable monomers for polymerizing-on the graft shell are in particular monomers and mixtures of these which form hard polymers or copolymers with glass transition temperatures above +50xc2x0 C. The type of monomer(s) depends here to a large extent on the type of the thermoplastics which form the polymer matrix after blending with the graft rubber and with which the graft shell should have some degree of compatibility or affinity, in order to achieve a fine two-phase distribution of the graft rubbers in the matrix. Particularly suitable and usual monomers are those having from 8 to 12 carbon atoms, for example, styrene, xcex1-methylstyrene, and also styrenes and a-methylstyrenes which have one or more alkyl substituents, in particular methyl substituents, on the benzene ring. They may be the sole monomers for preparing the graft shell, or be used in a mixture with other monomers, such as methyl methacrylate, methacrylonitrile or preferably acrylonitrile, in which case the proportion of methacrylonitrile monomer units and/or acrylonitrile monomer units in the graft shell is from 0 to 45% by weight, preferably from 10 to 40% by weight, of the graft shell. Preference is given to mixtures of styrene with from 10 to 40% by weight of acrylonitrile, based on the total amount of monomers. Other preferred monomers which may be mentioned for preparing the graft shell are methacrylates and acrylates, preferably methyl methacrylate, which may also be used as sole monomer or as the quantitatively predominant monomer for preparing the graft shell. Other suitable comonomers for preparing the graft shell are maleic anhydride, maleimide, N-phenylmaleimide, acrylic acid and methacrylic acid.
Examples of the preparation of dispersions of this type suitable for the application of shear precipitation are described, for example, in DE-C-2 60 135, DE-A-3 22 75 55, DE-A-3 14 93 57, DE-A-3 14 93 58 and DE-A-3 41 41 18, which are expressly incorporated herein by way of reference. However, these are in the nature of examples. The application of the shear precipitation according to the invention is not restricted to the examples of dispersions mentioned here.